Sorry my cat stepped on my mouse.... and hit reply before I was done...
I did a test on my ulcery horse because I wasn't sure what his problem was. He was definitely not as drastic as yours but maybe you could try the same test, if he seems better, then get the vet.
I soaked alfalfa cubes to mush and gave him several pounds every night. (He has TMJ and can't eat them dry.) He also got 1 oz Pro CMC with am and pm feed for 1 month. I noticed a dramatic difference. I had to take him off the alfalfa as it was just making him too hot so he is on the Pro CMC now.
His symptoms were the spooking but definitely not as bad as yours. Just ever so slightly and really not him so that is why I did this test. He had come off a treatment with that Doxy stuff for Lyme and I hear that is can cause some ulcery situations.
I am watching my guy and have conveyed all the info to my vet via e-mail. If he reverts to any traumatic signs I will call the vet and do that gastro guard route but for now, this works for me.
In January when I really don't ride and if he's a little hot it won't matter, I'll put him back on the alfalfa but I'll do Lucerne or something as I can't soak alfalfa cubes here in January...Or maybe purchase actual bales for the evening feed.